Directed self-assembly (DSA) is a technique in which diblock copolymers (BCP) containing dissimilar and non-intermixing blocks self-segregate into domains of homogeneous blocks. These domains may yield random patterns or, when directed, give well-defined and highly regular structures dictated by the molecular weight of each block. The ability of DSA to provide very small (sub-20-nm features) has quickly moved this technology into consideration as a viable option for integrated circuit production and semiconductor manufacturing processes.
DSA is also being investigated as a method for preparing nano-structured surfaces with unique surface physical properties. Possible applications include changing the hydrophobicity of surfaces due to incorporation of nano-structures and providing sites for unique chemical catalysts. DSA has promising applications in biomedical areas, including: drug delivery; protein purification, detection, and delivery; gene transfection; antibacterial or antifouling materials; and cytomimetic chemistry.
The ability to self-assemble is dependent on the Flory-Huggins Interaction Parameter (x). Higher values of x allow for lower molecular weight polymers to assemble, leading to smaller block domains and hence feature sizes, since the natural feature pitch (Lo) of lamellae-forming diblock copolymers is proportional to the degree of polymerization. It also allows for greater thermodynamic driving force to direct assembly onto either physically or chemically differentiated surfaces. To meet the needs of applications such as magnetic storage and semiconductor devices, many recent efforts have been aimed at achieving long-range ordering, good feature registration, and accurate pattern placement with very few defects. For example, a thin film of polystyrene/poly(methyl methacrylate) diblock copolymers can be spin-cast from a dilute toluene solution, then annealed, to form a hexagonal array of poly(methylmethacrylate) cylinders in a matrix of polystyrene (K. W. Guarini et al., Adv. Mater. 2002, 14, No. 18, 1290-4). Patterns of parallel lines have also been produced using PS-b-PMMA on chemically nanopatterned substrates (S. O. Kim et al., Nature, 2003, 424, 411-4).
Although there have been reports of using blends of diblock copolymers with the corresponding homopolymer(s) in forming patterns via directed self-assembly (e.g., US 2008/0299353), it is believed that there could be advantages in using block copolymers that are substantially free of homopolymer contaminants so that the composition of such blends can be more precisely controlled. However, it can be quite difficult to achieve the desired level of purity of the diblock copolymer without resorting to complex time- and resource-intensive procedures or sacrificing yield. Examples of attempts to achieve this desired end result are disclosed in U.S. Pat. No. 7,521,094; US 2008/0093743; US 2008/0299353; US 2010/0294740; and WO 2011/151109. However, none of these procedures produced a product suitable for DSA applications.
Therefore, there remains a need for scalable processes for separating homopolymer contaminants from the corresponding diblock copolymer.